Electrode and coating therefor

ABSTRACT

An electrode for use in an electrolytic reaction. The electrode has an electrically conductive base, preferably of a film-forming metal, the outside of which is a conductive material other than a film-forming metal, such as a layer of oxide of the base metal, and which is resistant to the electrolyte. At least a portion of the surface of said base has a coating of a mixed crystal material consisting essentially of at least one oxide of a filmforming metal and at least one oxide of a platinum group metal.

United States Patent [1 1 Beer [ ELECTRODE AND COATING THEREFOR [75]Inventor: Henri Bernard Beer, Kalmthout,

Belgium [73] Assignee: Chemnor Aktiengesellschaft, Vaduz,

Liechtenstein The portion of the term of this patent subsequent to Jan.4, 1989, has been disclaimed.

[22] Filed: May 19, 1971 [21] Appl. No.: 144,906

Related U.S. Application Data [62] Division of Ser. No. 702,695, Feb. 2,1968, Pat. No.

[ 1 Notice:

[30] Foreign Application Priority Data Feb. 10, 1967 Great Britain6,490/67 [52] U.S. Cl. 117/230, 117/201, 204/290 F [51] Int. Cl 801k3/04, B011: 3/06 [58] Field of Search 117/230, 201;

[ *Aug. 7, 1973 [56] References Cited UNITED STATES PATENTS 3,616,44510/1971 Bianchi 204/290 F 3,234,110 2/1966 Beer 204/290 F 3,645,8622/1972 Cotton 204/290 F Primary ExaminerRalph S. Kendall AssistantExaminer-M. F. Esposito Attorney-Wenderoth, Lind & Ponack [57] ABSTRACTAn electrode for use in an electrolytic reaction. The electrode has anelectrically conductive base, preferably of a film-forming metal, theoutside of which is a conductive material other than a film-formingmetal, such as a layer of oxide of the base metal, and which isresistant to the electrolyte. At least a portion of the surface of saidbase has a coating of a mixed crystal material consisting essentially ofat least one oxide of a film-forming metal and at least one oxide of aplatinum group metal.

9 Claims, 2 Drawing Figures PAIENIWUF 3.751.296

OVERPOTENTIAL mV TIME IN HOURS 7 HENRI BERNARD BEER,

INVENTOR.

702,695 filed Feb. 2, 1968 now US. Pat. No.

This invention relates to an electrode for use in an 5 electrolyticprocess, particularly in the electrolytic production of chlorine andalkali metal in mercury cells and diaphragm cells, the electrolyticproduction of chlorates, hypochlorites, persulphates, perborates, theoxidation of organic compounds, fuel cells, desalination andpurification of water, galvanic processes, and cathodic protectionsystems. Further the electrode has a long life, a low overvoltage andcatalytic properties.

The invention also relates to processes for making the electrode, and tomethods of carrying out electrolyses employing the electrode.

Hitherto it has been believed that the best electrode for use as anodein many electrolytic reactions was a solid metallic electrode of a noblemetal, such as a metal of the group of the platinum metals. However, byreason of the cost of such metals and certain undesirable technicalproperties, such as undesirable overvoltage, poor mechanical propertiesand structural difficulties, ways have been sought to provide electrodesplated with a platinum metal.

In recent years there have been developed electrodes on the basis oftitanium and coated with a platinum metal, which have proved to besatisfactory for many uses. It has been found, however, that theelectrodes having a titanium base and coated with a platinum metaldeteriorate in use at a rate which, although not harmful in many kindsof electrolyses, nevertheless results in the necessity of replacing theelectrodes from time to time at considerable expense,

In addition, there are other processes, in which the products of theelectrolysis should preferably not be contaminated with the materialgiven off by the electrodes. If such a material is present in theproducts of the electrolysis, it must be removed by a separatetreatment.

It is one object of the present invention to provide an electrode foruse in electrolytic processes, whereby to eliminate substantially thedisadvantages of the prior electrodes, said electrode being inexpensiveand easy to manufacture.

It is another object of the invention to provide such an electrode whichutilizes relatively inexpensive metal in the coating thereon, and whichis nevertheless excellent for carrying out electrolytic processes, has along life, and is stable in operation.

Still another object of the invention is to provide processes for makingthe electrode, methods for the use of the electrode and for carrying outelectrolytic processes employing the electrode.

These and other objects are achieved, according to the invention, by anelectrode based on the discovery that when the electrode comprises aconductive base with a coating consisting essentially of a combinationof one or more oxides of one or more film-forming metals with one ormore non-film-forming conductors, there is obtained an electrode havingexcellent characteristics of resistancy, durability and efficiency.

By film-forming metals are understood metals which when connected as ananode in an electrolyte form an oxide coating on their surface whichseals off the subjacent metal in such a manner as to practically bar thepassage of current after a period of a few minutes.

By non-film-forming conductors are understood conductors which whenconnected as an anode is an electrolyte continue to transport thecurrent into or out of the electrolyte practically without losses.

The invention will now be described in greater detail with reference tothe accompanying drawings, in which FIG. 1 is a cross-section of theelectrode according to the invention; and

FIG. 2 is a graph in which the performance of prior electrodes iscompared with that of an electrode according to the invention.

Referring to the drawings, the electrode according to the inventionconsists of a base or core 10 having a coating 11 thereon, which twoparts consist of materials which will be described more fullyhereinafter.

The electrode is shown as having a simple rectangular shape, but it willbe understood that the electrode is notlimited to such a configuration,but may have any configuration suitable for the electrolysis apparatusin which the electrode is to be used. Furthermore, there is shown asimple cavity 12 at the top for connecting the current conductorthereto, but this feature does not constitute part of the invention andmay be changed as desired.

The base or core of the electrode according to the invention consistsof. a conductive material which at least on the outside is resistant tothe electrolyte in which it is to be used. Thus, for example, the basemay consist of any of the film-forming metals, such as, aluminum,tantalum, titanium, zirconium, bismuth, tungsten, nio' bium, or alloysof two or more of these metals. However, I may use other conductivematerials which will not be affected by the electrolyte and the productsformed during the dissociation thereof, it being possible to use metalssuch as iron, nickel or lead, and nonmetallic conductive materials, suchas graphite, in suitable electrolytes.

It is an essential feature of the coating 11 that is behaves as amixed-crystal material which contains one or more oxides of one or moreof the film-forming materials set out hereinbefore, and preferably morethan 50 mol percent of such an oxide or oxides. By mixedcrystal materialis generally understood that the molecular lattices of the oxide of thefilm-forming metal are intertwined with the molecular lattices of theother material constituting the coating. There are various methods ofachieving such a structure, some of which will be described hereinafterin connection with the processes for making the electrode according tothe invention, but this is not intended to restrict the scope of theinvention.

The other material of the mixture consists of one or morerepresentatives of the non-film-forming conductors. This other materialmay consist of a mixture of a metal and the oxide of the metal, or of amixture of two metals, or of a mixture of a metal and an oxide of adifferent metal, or other permutations and combinations of conductorsand oxides. Preferably the conductors belong to the group consisting ofgold, silver, platinum, palladium, iridium, ruthenium, osmium, rhodium,iron, nickel, chromium, copper, lead, manganese, and the oxides thereof,graphite, nitrides, carbides, and sul-,

The coating according to the invention need not cover the entire surfaceof the electrode to be immersed in the electrolyte. As a matter of fact,the coating need only cover 2 percent of the immersed zone,

and the electrode will still operate effectively and efficiently.

There are a number of methods of forming the coating on the base toproduce the mixed-crystal material. The most practical one thereofcomprises the coprecipitation of an oxide of a film-forming metal withthe other material of the mixture constituting the coating, whichcoprecipitation may be effected chemically, thermally, electrically, orby a combination of these methods. One method of effecting such acoprecipitation consists in preparing a solution containing materialsfrom which one or more oxides of the film-forming metal can beprecipitated, and further materials from which non-film-formingconductors can be precipitated and thereafter treating the solutions insuch a manner that the oxide or oxides of the film-forming metal arecoprecipitated with the conductors of the non-filmforming type. Amongthe methods of treating the solution are evaporation of the solventfollowed by the thermal formation of the mixed crystals, whereby, whenthe solution is first applied to the surface of the electrode to becoated, by a treatment such as brushing, immersion, or spraying, thecoprecipitated mixture remains behind on the surface of the electrode.Alternatively, the acidity of the solution can be so adjusted that thematerials of the mixture are precipitated to form a suspension and thenthe portion of the electrode to be coated can be immersed in thesuspension and an electrophoresis effected to precipitate the materialsonto the electrode. Such a method is preferably followed by sintering topromote the adhesion of the deposited mixture of the material of thecore of the electrode.

A particular method of co-precipitating the materials to form themixed-crystal material comprises preparing a solution containing asolvent and a soluble compound or compounds of a film-forming metal,which will precipitate when the solvent is evaporated, and a solublecompound or compounds of a non-film-forming conductor, which will alsoprecipitate when the solvent is evaporated. The solution is applied tothe surface of the electrode base to be coated, and the base thus coatedis heated one or more times, preferably several times, in a non-reducingatmosphere.

Alternatively, only one of the materials in the solvent need beevaporated, that is to say, either a compound from which an oxide of afilm-forming metal can be deposited, or a compound from which anon-film-forming conductor can be deposited, the other compound orcompounds being suspended in the solution. The subsequent treatments arethe same as in the case that all materials are in the dissolved state.

A different method of making the electrode consists in the use of theso-called vacuum-sputtering techniques, in which the base is placed in avacuum and connected as a cathode, and anodes of one or morefilm-forming metals, are placed in the vacuum together with an anode ofan electrolytic non-film-forming metal or an oxide thereof, or anodes ofelectrolytic non-filmforming metals or oxides thereof, and thesputtering current is conducted through the anodes and the cathode sothat the electrolytic film-forming metal oxide or oxides are sputteredonto the cathode together with the electrolytic non-film-forming metalor metals or oxide or oxides thereof.

Still another method of making the electrode accord ing to the inventionconsists in the use of an electrolysis. The base of the electrode isimmersed in an electrolyte consisting of a solution of salts or othercompounds of one or more film-forming metals, from which solution theoxide or the oxides will co-precipitate onto the electrode when thesolution is subjected to electrolysis. The solution also contains asoluble compound of a non-film forming metal or metals or of an oxide oroxides of such metals which will also co-precipitate during theelectrolysis. The electrolysis can be effected either by passing analternating current through the electrode, or by using the electrode asan anode and conducting a direct current through it.

Generally speaking, the formation of the mixtures of the oxidesaccording to the invention can be effected thermally by heating in theair, but in some cases this can be beneficially affected by conductingthe heat treatment under sub-atmospheric or super-atmospheric pressure.The heating may be effected by resistance heating or high-frequencyheating.

When the mixtures are applied alectrolytically, this is best effectedunder anodic conditions, and preferably so that one or more hydroxidesof the metals are deposited on the base, such hydroxides beingsubsequently sealed by boiling in demineralized water or by heating.

Generally speaking, the starting products are salts of the metals, whichare converted into the desired oxides thermally. The acid residue ispreferably so selected that the salt is converted into an oxide at atemperature of from 400l ,200C. l preferably use acid residues ofvolatile acids, such as HCI, HBr, or acetic acid.

The manner in which the electrode according to the invention is usedwill be readily apparent to those skilled in the art. For most uses, theelectrode is placed as an anode in an electrolysis apparatus, and theelectrolysis is carried out in the conventional manner and underconventional conditions, the product or products of the electrolysisbeing yielded in the conventional manner, or the purified electrolytebeing recovered, as desired. Examples of processes in which theelectrode is thus used are the electrolysis of brine in mercury cells ordiaphragm cells for the production of chlorine and alkali metal, theelectrolytic production of chlorates,

hydrochlorites, persulphates, and perborates, the electrolytic oxidationof organic compounds, such as liquid or gaseous hydrocarbons, forexample, propylene or ethylene, the electrolytic deposition of metals,desalination of water, sterilization of water, and fuel cells. Theelectrode is also excellently suitable for use as an anode in cathodeprotection systems and as a cathode in bi-polar cells.

As explained above, the provision of the mixedcrystal coating is theparticular feature accounting for the outstanding performance of theelectrode according to the invention. The importance of the restrictionthat the coating must behave as a mixed-crystal material rather than asa mere mixture of the two oxides can be shown by means of severalexamples. Iron oxide itself is highly sensitive to hydrochloric acid atroom temperature, and so are several titanium oxides. It has been found,however, that when a co-precipitated mixture of iron oxide and titaniumoxide is applied to a basis of conductive material it is only affectedby hydrochloric acid at room temperature to a very small ex tent.Similarly, ruthenium oxide coated on a titanium base, connected as ananode in an alkali metal chloride electrolysis, which anode is contactedwith the amalgam formed in a mercury cell, loses a part of its thickncssafter a prolonged period of electrolysis, because the reductiveproperties of the amalgam convert the ruthenium oxide into metallicruthenium, and the metallic ruthenium is readily dissolved in theamalgam from the surface of the titanium and is not resistant to theelectrolyte. Co-precipitated mixed oxides of titanium oxide andruthenium oxide, however, which are in contact with such an amalgam areresistant to the amalgam because these oxides when in mixed-crystal formare not reduced and so do not dissolve in the amalgam or in thegenerated chlorine.

It should be noted that the mixed crystals which are applied to theelectrodes according to the present invention are quite different fromthose obtained, for example, by mere heating in the air of the solidnoble metals, or when these are superimposed in discontinuous layers infinely divided condition on other metals. Generally speaking, it may besaid that the oxidation of the solid metals by mere heating is verydifficult, and that, although finely divided noble metals may beoxidized, the adhesion of such oxides to the substrate is often verypoor. Electrolytic oxidation is also very difficult, and in additionlayers produced in this manner also show poor adhesion, so that amechanically weak electrode is formed. The problem of rendering theoxides of the noble metals and other metals in finely divided conditionadhesive and at the same time resistant is now solved by virtue of theco-precipitation of the non-film-forming conductors with the oxides ofthe film-forming metals. It is surprising, for example, that palladiumoxide, platinum oxide, and ruthenium oxide are then fully resistant.Therefore, it is decidedly not so that platinum, applied to a metallicbase, when heated in the air or used as an anode in the electrolysis ofalkali metal chloride, just acquires the condition required according tothe present invention, that is to say, that an adhering mixture isco-precipitated thereon.

The following Table A clearly indicates the difference between theco-precipitated oxides according to the present invention and the otheroxides which may be formed thermally or electrolytically when, forexample, an electrode consisting of a base of metallic titanium and acoating of a platinum metal is oxidized in the air or used toelectrolyse a dilute solution of an alkali metal chloride or dilutehydrochloric acid.

Chemical and electrolytic properties of singular oxides compared withthe mixed oxides according to the invention Thennal oxidation in air at500C of Pt/Pd/AgIFe/Ru in finely-divided condition on titanium baseFormation of oxide B/B/B/B/G Adhesion to base metal BlB/BIB/B Resistanceto 0.2% sodium amalgam B/B/B/B/B Overvoltage in chlorine electrolysis at8,000 amps/m B/B/l/B Loss of oxides per ton of chlorine at 8,000 amps/mM/M/-//M Chemical resistance 'to aqua regia without current B/B//-/BResistance to reduction B/B/BIB/B Catalytic properties in the oxidationof organic compounds B/B/B/B/B Mechanical strength B/B/B/B/B Key: Eexcellent B bad G good N practically no oxide formed M much L verylittle Electrolytic oxidation in dilute sulphuric acid of Pt/Pd/Ag/Fe/Ruin finely-divided state on titanium Co-precipitated oxides of Ru/T i,

PtIZr, Pd/l a, Ag/T i, Fe/T i, and Ptfli in finely-divided state ontitanium base.

Formation of oxide E/E/E/E/E/E Adhesion to base metal EIE/E/E/E/EResistance to 0.2% sodium amalgam E/E/El-lE/E Overvoltage in chlorineelectrolysis at 8,000 amps/m E/E/El-//E Loss of oxides per ton ofchlorine at 8,000 amps/m L/L/Ll-l-IL Chemical resistance to aqua regiawithout current E/E/E///E Resistance to reduction E/E/El-l-lE Catalyticproperties in the oxidation of organic compounds E/E/E//-/E Mechanicalstrength E/ElE///E In a test designed to show quantitatively theimprovement obtained by electrodes according to the invention ascompared with other electrodes in contact with 0.2 percent sodiumamalgam under a constant electric load of 10.000 Amp/m duringelectrolysis and of 80.000 Amp/m during short-circuiting with theamalgam, a

' number of titanium bases were respectively coated with (l) metallicruthenium, (2) a mixture of platinum and iridium (/30 weight/weight),(3) a co-precipitated mixture of ruthenium oxide and titanium oxide (90mol 10 mol and (4) a co-precipitated mixture of ruthenium oxide andtitanium oxide (30 mol 70 mol All these materials were present in athickness of 10 g/m. Thev electrodes were introduced into a brine testcell containing 0.2 percent sodium amalgam, which quantity was keptconstant, the brine in the cell having a concentration of 28 percent anda temperature of C, the applied current density being 10.000 Amp/m Theseconditions are the same as may be the case in a large-scale cell. FIG. 2shows the overvoltage in millivolts plotted against the time. It will beseen that, whereas the overvoltage of the first, second and thirdelectrodes increased relatively rapidly owing to the contact with theamalgam, the overvoltage of the electrode according to the invention,.i.e., containing more than 50 percent titanium oxide, only increasedgradually during a long period of time.

The invention is illustrated, but not limited, by the followingexamples.

EXAMPLE I 6.2 cc butyl alcohol 0.4 cc HCl 36 percent 3 cc butyl titanateg RuCl The solution was several times brushed on 'to a cleaned titaniumplate (grain size titanium 0.04-0.06 mm; ASTM 6) 'of 10 -X 10 cm'and athickness-oft mm, the plate being first pickled in hot aqueousoxalic-acid,

subjected to ultrasonorous vibration in water, and dried. The plate thustreated was heated in the airat a temperature of 300 500C for l minutes.

The resulting electrode had a coating of ruthenium oxide coprecipitatedwith titanium oxide, the titanium oxide being present in a proportion of70 mol the balance being RuO The resulting electrode was placed in ahydrochloric acid cell as an anode, the cathode being a silver platedtitanium electrode. Flowing hydrochloric acid of 25 percent waselectrolyzed at 70C and 2,500 Amp/m for practically one year withexcellent results and with losses of less than 0.1 g ruthenium per tonof chlorine.

The resulting electrode was placed in a brine electrolysis cell as ananode, the cathode being mercury and the brine having a concentration of28 percent a pH of about 2.5, and a temperature of 80C. The spacingbetween anode and cathode was less than 2.5 mm. When a current densityof 10.000 Amp/m was applied, the anode had an extremely low overvoltageof about 80 millivolts, measured against a calomel reference electrode,and this was maintained for a long period of time, even after variousshort-circuitings with the amalgam.

The resulting electrode was placed in a brine diaphragm cell as ananode, the cathode being iron, and the brine having a concentration of28 percent, a pH of about 3.5, and a temperature of 80C. At a currentdensity of 1,000 Amp/m the anode had an extremely low overvoltage of 60mVolts and maintained this for a long period of time. The losses ofmetallic ruthenium were less than 0.15 g per ton of produced chlorine,in the mercury cell and less than 0.1 g of produced chlorine in thediaphragm cell.

The resulting electrode was also used in a cathodic protection system asanode for the protection ofa ship. The electric design was aconventional system wellknown to those skilled in the art. The anodeshowed good electrical and mechanical properties.

The resulting electrode was extremely suitable for the oxidation ofunsaturated organic compounds, such as ethylene and propylene, as wellas for the preparation of chlorates.

The resulting electrode was also suitable for electrodialysis, becauseit readily admits of pole changing.

The resulting electrode was also used in a galvanic metal depositionprocess, in which gold was deposited on copper from a bath having thefollowing composition: gold chloride 30 g/l, nitric acid (specificgravity 1.19) 25 cc/l, sodium chloride 12 g/l, sulphuric acid (specificgravity 1.025) 13 g/l organic brighteners). By means of this bath, at70C, and a current density of 8-10 Amp/m an excellent plating on thecathode was obtained, the overvoltage at the anode being such that nodamage was done to the bath.

EXAMPLE ll 80 cc TiCl -solution in H 0 (25% TiO,)

1 g RuCl,

This mixture was absorbed in a graphite anode at a subatmosphericpressure this anode being previously subjected to ultrasonorousvibrations for minutes. Subsequently the anode was heated in a stream ofair for one-half hour at a temperature of 300 800C. This treatment wasrepeated four times. The resulting electrode had a coating of rutheniumoxide, coprecipitated with titanium oxide, the titanium oxide beingpresent in a proportion of 98.4 mol TiO there being 1.6 mol RuO Anuntreated graphite anode was placed in an alkali metal chloride cellcontaining 28 percent brine of a pH of about 2.5 and a temperature of80C as the electrolyte and a mercury cathode. The distance between theanode and the cathode was less than 2.5 mm.

A current of a density of 8.000 Amp/m was passed through the cell. Theanode first had an overvoltage of about 400 mV, which decreased to 360and after a considerable time increased to 450 mV. Furthermore, theuntreated anode showed marked erosion after a short while, and as aresult the brine solution became black with the graphite released. Inaddition to the contamination of the bath liquid, the loose graphitecaused stray currents, resulting in loss of efficiency and discharge ofthe amalgam. Furthermore, the spacing between the anode and the cathoderequired adjustment at regular intervals, because this spacing changedas a result of the erosion of the anode, resulting in loss of energy inthe electrolyte.

The electrode according to this example, placed in the same electrolyteunder the same conditions, had an overvoltage of only mV, whichovervoltage remained constant during a considerable time. Moreover, thebath remained clear and the anode showed no erosion. Accordingly, notonly was the electrolyte not contaminated, but the electrodes did notrequire adjustment.

The electrode according to the invention was also used as an anode in acathode protection system of a conventional type and operatedexcellently.

EXAMPLE III A tantalum plate was cleansed well and mechanicallyroughened, and a coating mixture was prepared as follows:

l8 cc isopropyl alcohol 1 g iridium chloride 2 g platinum chloride 4 gisopropyl titanate 3 cc anise-oil (reducing agent) Lavender oil orlinalool may be used instead of the anise-oil.

The mixture was brushed onto the tantalum plate several times, and thecoated base was subsequently heated at a temperature of 600C for severalminutes. The resulting electrode had an oxide coating of iridium andplatinum, co-precipitated with titanium oxide, the titanium oxide beingpresent in a proportion of 65.8 mol percent in addition to 12.65 molpercent iridium and 21.55 mol percent platinum.

This electrode operated excellently in electrolytic processes for thepreparation of chlorine, oxygen, oxi dation of organic compounds, and ingalvanic baths.

EXAMPLE IV A zirconium plate was degreased and a coating mixture wasprepared as follows:

10 cc water 1 g gold chloride 3 cc 25 percent titanium chloride solution0.1 cc wetting agent This mixture was brushed onto the degreased plateand the plate was heated in the air at a temperature of 200 300C and ata superatomspheric pressure. This treatment was repeated eight times.

The resulting electrode had a coating of gold oxide coprecipitated withtitanium oxide, the titanium oxide being present in a proportion of 74mol percent and the gold oxide in a proportion of 26 mol percent.

This electrode operated excellently in dilute sulphuric acid solutions.

EXAMPLE V EXAMPLE VI 6.2 cc butyl alcohol 0.4 cc hydrochloric acid 36percent 1 g zirconium acetyl acetonate l g iridium chloride, dry

The solution was applied to a zirconium base as described in Example I,the base being previously de- 3 greased, pickled, and subjected toultrasonorous vibrations. After the application of the solution the basewas heated at 500 700C for several minutes by clamping the base betweentwo copper plates heated throughout their surface. This resulted in ahighly'uniform heating of the overall surface, which was highlybeneficial to the quality of the anode. The treatment was repeatedseveral timesThe ratio. of zirconium oxide to iridium oxide in themixture had been so selected that more than 50 mol percent of zirconiumoxide was present in it. The anode thus made was excellently suitablefor all kinds of electrolytic processes, particularly for theelectrolysis of sulfuric acid solutions and solutions of sulfates.

EXAMPLE VII 9 cc butyl alcohol 0.4 cc hydrochloric acid 36 percent 1 gpalladium chloride 3 cc pentaethyl tantalate.

A tantalum base was dipped into the above solution and after dryingheated at 500 800C to deposit a mixture thereon of 62 mol tantalum oxideand 38 mol palladium oxide. This treatment was repeated six times. Thetantalum base was a thin tube which after the completion of the coatingwas provided with a copper rod acting as a current conductor, becausethe tantalum tube comprised insufficient metal for it to be able totransport current without undue losses. In order to ensure propercontact between the tantalum tube and the copper rod, the inner surfaceof the tantalum tube was electrolytically copper-plated. Intimatecontact between the copper rod and the copper inner coating was obtainedby applying molten tin therebetween and allowing the tin to' solidify.

The anode made in this manner was excellently suitable for cathodicprotection purposes with an applied voltage of higher than 20 volts, andalso is an excellent anode for the preparation of hypochlorites.

EXAMPLE VIII 6.2 cc butyl alcohol 0.4 cc hydrochloric acid 36 percent 1g ruthenium chloride 3 cc niobium pentaethylate.

A niobium base was degreased and connected as an anode in an electrolyteto form an oxide coating thereon. This coating was subsequently rinsedthoroughly and dried. The anode with the oxide coating thereon wasdipped into the above solution and subjected to high-frequency heatingat 600C at a subatmospheric pressure of mm Hg to convert the reactantsto the desired mixture. This treatment was repeated several times untilthe desired mixture was present on the niobium in a thickness of 2microns.

The anode thus made was excellently suitable for all kinds ofelectrolytic processes, such as for the'preparation of chlorine,chlorates, and hypochlorites, for the sterilization of swimming-pools,etc.

EXAMPLE IX A titanium plate was degreased and pickled and subsequentlyan oxide coating of about 1 mm thickness was applied to it by means ofelectrolysis.

A mixture of:

10 cc butyl alcohol I g ruthenium oxide powder 3 cc butyl titanate waspainted onto it and converted into the desired mixture at a temperatureof 300 600C. This treatment was repeated so many times that l0g/m of thedesired mixture was present on the surface of the titanium plate.

I The anode made in this manneer was excellently suitable for theelectrolytic preparation of chlorine, and chlorine compounds, and forcathodic protection purposes.

The electrolytically formed oxide on the titanium highly promotes theadhesion of the mixture formed.

EXAMPLE X A niobium expanded metal plate was pre-treated in known mannerand subsequently brushed with a solution of:

10 cc water I g ruthenium chloride cc hydrochloric acid (35 percent) 2 gtitanium hydroxide.

The plate was subsequently heated at 400 700C for several minutes untilthe desired mixture formed. This treatment was repeated until figlmofthe mixture was present on the surface.

This anode was excellently suitable for the electrolysis of alkalinesolutions.

EXAMPLE XI An aluminum plate was degreased and pickled in a conventionalmanner. There was then prepared a mixture of:

10 cc isopropyl alcohol I g aluminum bromide l g platinum chloride 0.01g iodine.

ill

The aluminum plate was dipped into this mixture and heated at 400C toform the required mixture, the latter consisting of 62.2 mol A1 and 37.8mol PtO This treatment was repeated several times, the mixture beingapplied to the plate either by dripping or painting.

The electrode thus made is excellently suitable for the electrolysis ofboric acid compounds.

EXAMPLE XII The following mixture was prepared:

cc butyl alcohol 6 cc butyl titanate 2 g graphite (can be replaced bytitanium nitride or tantalum carbide or rhenium sulfide) This mixturewas painted onto a titanium base and heated at a temperature of 400700C, which treatment was repeated a number of times.

An anode thus coated with graphite and titanium oxide is particularlysuitable for electrolyses in which a low current density is desirable,for example, cathodic protection of subterraneous objects.

Anodes in which the coating contains in addition to titanium oxide anitride, carbide; or sulfide are resistant to high current densities invarious electrolytes.

EXAMPLE XIII 2 g titanium chelate l g ruthenium chelate These twochelates were intimately admixed in the dry state and subsequentlyplaced on the bottom of a vessel which can be closed and heated. Adegreased, pickled titanium rod, covered as to 98 percent with aheat-resistant silicon lacquer layer, was introduced into the vessel. Byheating the chelates, a mixture of titanium oxide and ruthenium oxidewas evaporated onto the 2 percent of exposed titanium, and the requiredcrystal form was obtained by sintering. A small quantity of hydrochloricacid vapour in the vessel promotes the adhesion of the mixed oxide.Subsequently the lacquer layer was removed. The resulting electrode hasan active surface area of about 2 percent.

This electrode is excellently suitable as an anode for the sterilizationof water in swimming-pools or for the electrolysis of two layers ofliquid, in which a local electrolysis of either of the liquids isdesired.

Naturally, partly coated anodes may also be made in different mannersfrom that described in this example.

EXAMPLE XIV 10 cc butyl alcohol 2 cc butyl titanate 1 cc pentaethyltantalate 1 cc pentaethyl niobate l g ruthenium chloride, bromide, oriodide 0.1 g hydrogen chloride A zirconium base was degreased andpickled in known manner. The above mixture was painted onto the base andconverted by heating at 400 700C in air. This treatment was repeateduntil 40 g/m of the desired mixture was present on the surface. Themixed crystal consisted of the oxides of titanium, tantalum, and niobiumas oxides of film-forming metals and ruthenium oxide as the oxide of anon-film-forming conductor.

In addition, some zirconium oxide had formed thermally on the boundarysurface of the mixture and the zirconium rod. The quantity of oxides offilm-forming metals was more than 50 mol calculated on the overallmixture.

Such an anode is particularly suitable for all kinds of electrolyses,such as of sulphuric acid compounds, for the purification of water, andfor the preparation of chlorates.

EXAMPLE XV A titanium plate was-degreased, pickled, and subjected toultra-sonorous vibrations. Subsequently the plate was placed as anelectrode in a stirred emulsion consisting of:

100 cc water 100 cc acetone 5 g extremely finely divided mixture ofcoprecipitated platinum oxide (3 g) and titanium oxide (2 l gemulsifying agent The second electrode was constituted by a platinumplate. By applying an electric voltage of l0 100 volts, the titanium waselectrophoretically coated with a mixed oxide from the emulsion. Afterbeing removed from the bath, the titanium with the coating depositedthereon was carefully dried and subsequently heated at 400C for severalminutes. Thereafter the electrophoretically deposited layer had anexcellent adhesion to the titanium, and the anode thus made is suitablefor various kinds of electrolyses.

The adhesion is highly promoted by pre-oxidizing the titanium base bymeans of heat or electrolytically, and then applying the mixed oxide byelectrophoresis.

This example was repeated using a mixture of coprecipitated platinumoxide, titanium oxide, and manganese dioxide. There is thus obtained ananode having a high overvoltage and catalytic properties.

EXAMPLE XVI Two titanium rods were degreased and pickled andsubsequently placed in a galvanic bath having the following composition:

100 cc ethanol 100 cc water 1 g ruthenium chloride 10 g titaniumchloride and subsequently connected to a source of alternating currentof 13 volts and a current density of 15 Amp/m, temperature 20 30C, for aperiod of time of about 20 minutes.

After about 20 minutes both rods were coated with a mixture of titaniumoxide and ruthenium oxide, the adhesion of which was still furtherimproved by heating at 400C for 5 minutes.

The anode thus made is excellently suitable for use in variouselectrolyses effected at low current densities.

EXAMPLE XVII A titanium rod was degreased and subsequentlyelectrolytically provided with an oxide coating of a thickness of about5 microns. The rod thus treated was placed as an anode in a bath (C),containing:

cc water 5 g yellow lead oxide 5 g sodium hydroxide 3 cc hydrogenperoxide 10 cc titanium chloride solution (25% TiO,)

This bath is regularly insufflated with air. The treated titanium rodwas connected as anode, an iron plate being used as the cathode. Thevoltage differential between the anode and the cathode was about 2 3volts, and the current density was about 5 Amp/m After about an halfhour, the titanium anode was coated with a mixture of titanium oxide andlead oxide, the properties of which could be considerably improved byheating at 200 600C.

An anode thus treated is suitable for use in electrolyses in which nohigh-current densities are necessary.

EXAMPLE XVlll Titanium expanded metal was degreased and pickled, andthen painted, with the following mixture:

cc butyl alcohol 1 g ruthenium chloride 3 cc zirconium acetyl acetonate.

Subsequently the product was heated at 400 700C. This treatment wasrepeated until the mixture on the titanium had a thickness of one-halfmicron.

An electrode thus made is excellently suitablefor the electrolysisofsolutions of sulphuric acid compounds, the resistance of the titaniumto sulphuric acid being greatly increased by virtue of the mixed surfacecoating containing zirconium oxide.

EXAMPLE XIX A tantalum wire was degreased and pickled, and then dippedinto a mixture of l0 cc butyl alcohol 3 cc butyl titanate l g iridiumchloride The wire was then heated at a temperature of 500 700C, and thetreatment was repeated until at least 2.5 g of the mixture of the oxidesper m was present on the surface of the tantalum.

A tantalum wire thus treated is excellently suitable for use as an anodefor the cathodic protection of ships.

EXAMPLE XX A titanium plate was degreased, pickled, subjected toultrasonoric vibrations and then rinsed thoroughly and dried. This platewas subsequently connected as a cathode in an apparatus in which metalscan be vacuum deposited. As anodes, a bar of platinum and bars oftitanium were connected, the ambient atmosphere containing so muchoxygen that the anodic materials were deposited on the titanium cathodeas oxides (a detailed description of this apparatus is contained in thebook by L. Holland, Vacuum Deposition, 1963, pages 454 458).

After several minutes a mixture of titanium oxide and platinium oxidehad deposited on the titanium, and the titanium thus treated isexcellently suitable for the electrolysis of aqueous electrolytes.

EXAMPLE XXI Niobium was degreased and subsequently provided with anoxide coating of a thickness of at least 1 'micron. This can be effectedeither electrolytically or thermally.

Subsequently a paste was prepared of:

10 cc ethanol 1 g ruthenium oxide 4 g titanium oxide.

This mixture was intimately admixed, heated, sintered, comminuted, andagain mixed with 10 cc ethanol. The resulting paste was applied to theoxidized niobium in a thin layerfand subsequently heated at atemperature of 450 700C. This treatment was repeated until at least 10 gof the desired mixture per m was present on the surface.

' A niobium plate thus treated is excellently suitable for theelectrolysis of electrolytes.

EXAMPLE XXII A soft-qualitytitanium rod was degreased and then a mixtureof more than 50 mol percent titanium oxide and less than 50 mol percentpalladium oxide was rolled into it under pressure. Alternatively, thismay be effected by hammering.

The oxides were prepared by dissolving water-soluble salts of the metalsin water in the required proportions, from which solution they wereprecipitated with lye, washed, and carefully dried. In this manner avery fine mixed oxide was obtained, which could be hammered or rolledinto the titanium without undue trouble. Other conventional methods ofpreparing these mixed oxides can naturally be used as well.

Furthermore, other mtals than titanium can naturally be treated in thismanner.

EXAMPLE XXIII 6.2 cc butyl alcohol 0.4 cc water 3 cc butyl titanate 1 gruthenium chloride.

The above solution was painted onto a titanium base and heated asdescribed in Example I.

An anode thus made is particularly suitable for the electrolysis'of zincsulphate or copper sulphate solutions, which may be contaminated withnitrate or chloride, for the manufacture of the metals concerned.

EXAMPLE .XXlV

A zirconium plate was degreased and subsequently provided with thedesired mixture of oxides by means of a so-called plasma burner. I

There is thus obtained a very thin, but excellently adhering layer, anda zirconium plate provided with such a coating is excellently suitablefor all kinds of electrolyses.

I claim:

1. An electrode for use in an electrolytic reaction comprising anelectrically conductive base selected from the group consisting ofaluminum, tantalum, titanium, zirconium, bismuth, tungsten, niobium andalloys thereof, at least a portion of the surface of said electrodehaving a coating of a mixed crystal material consisting essentially ofat least one oxide of a film-forming metal and at least one oxide of aplatinum group metal.

2. An electrode according to claim 1 wherein said oxide of afilm-forming metal is an oxide of ametal selected from the groupconsisting of aluminum, tantalum, titanium, zirconium, bismuth, tungstenand niobium.

3. An electrode according to claim 1 wherein said oxide of a platinumgroup metal is an oxide of a metal selected from the group consisting ofpalladium, platinium, rhodium, iridium, ruthenium and osmium.

'4. An electrode according to claim 1 wherein the coating containsoxides of a plurality of film-forming metals selected from the groupconsisting of aluminum, tantalum, titanium, zirconium, bismuth, tungstenand niobium.

5. An electrode according to claim 1 wherein the coating contains oxidesof a plurality of platinum group metals selected from the groupconsisting of palladium, platinum, rhodium, iridium, ruthenium andosmium.

6. An electrode according to claim 1 wherein the oxide of thefilm-forming metal is present in a proportion higher than 50 mol percentof the materials of the coating.

7. An electrode according to claim 1 wherein the coating covers at least2 percent of the surface of the electrode adapted to be placed in anelectrolyte.

8. An electrode according to claim 1 wherein the of 30 mol percent ofthe material of the coating.

' UNETED STAKES PATENT @FFRIE QETEHQAEE 0F CQRREC'ESWN Patent No.3,751,296 Dated August 7, 1973 Inventor HENRI BERNARD BEER It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In the heading, change "Chemnor A.G.,, Vaduz, Liechenstein" to -ChemnorCorporation, Panama City, Panama-.

'ignm} and sealed this 18th day of February IQZQ,

f v-u 'r 0 attest:

I T-LXRSHALL DAN'N RUTl- C. FLA-SON Commissioner of Patents attestingOfficer and Trademarks- 1 -s ORM PO 1050 0 9) USCOMM-DC 60376-P69 Q ".5.GOVERNMENT PfiNYING OFFlCE 9G9 0-35633

2. An electrode according to claim 1 wherein said oxide of afilm-forming metal is an oxide of a metal selected from the groupconsisting of aluminum, tantalum, titanium, zirconium, bismuth, tungstenand niobium.
 3. An electrode according to claim 1 wherein said oxide ofa platinum group metal is an oxide of a metal selected from the groupconsisting of palladium, platinum, rhodium, iridium, ruthenium andosmium.
 4. An electrode according to claim 1 wherein the coatingcontains oxides of a plurality of film-forming metals selected from thegroup consisting of aluminum, tantalum, titanium, zirconium, bismuth,tungsten and niobium.
 5. An electrode according to claim 1 wherein thecoating contains oxides of a plurality of platinum group metals selectedfrom the group consisting of palladium, platinum, rhodium, iridium,ruthenium and osmium.
 6. An electrode according to claim 1 wherein theoxide of the film-forming metal is present in a proportion higher than50 mol percent of the materials of the coating.
 7. An electrodeaccording to claim 1 wherein the coating covers at least 2 percent ofthe surface of the electrode adapted to be placed in an electrolyte. 8.An electrode according to claim 1 wherein the electrically conductivebase is selected from the group consisting of titanium and tantalum, andthe surface of said electrode has a mixed crystal material coatingconsisting essentially of titanium oxide and ruthenium oxide.
 9. Anelectrode according to claim 1 wherein said oxide of a film-formingmetal is titanium oxide and is present in a proportion of 70 mol percentof the material of the coating, and said oxide of a platinum group metalis ruthenium oxide and is present in a proportion of 30 mol percent ofthe material of the coating.